Vacuum interrupters have been widely available in the prior art, the principal use being in high power utility and industrial applications. The inherent advantages of equipment using vacuum dielectric interrupters over other types include high interrupting speed, high speed operation, rapid dielectric recovery, reduction in size and weight, quiet operation, minimal maintenance requirements, long life and economy.
Vacuum interrupter switches have demonstrated superior performance for such RF applications as band switching of transmitters, switching of filter sections and antenna multicouplers, antenna reflector switching, tap changing of rf coils in induction and dielectric heating RF generators and switching of transmission lines. Most of these applications are in the RF band and involve currents ranging from 20 amperes to several hundred amperes.
Two typical prior art RF switches manufactured by ITT Jennings, San Jose, Calif., are the RP233 and the RF20 models.
The RP233 comprises fixed and moveable contacts mounted inside an evacuated ceramic-metal container. The fixed contact is mounted on a tube which forms part of the envelope structure. The moveable contact is mounted on the end of a bellows which also forms part of the envelope structure. A shaft and bearing arrangement position and guide the moveable contact. A compression spring on the outer end of the shaft holds the moveable contact open (away from the fixed contact). When air or other fluid pressure is applied to the inside of the bellows, it acts as an air cylinder and exerts a force which overcomes the compression spring and urges the moveable contact closed. Although this unit performs very satisfactorily, the RF current travels near the surface of the fixed contact tube and bellows. This fact results in (1) a large inductance in series with the switch contacts, and (2) large internal resistance in series with the switch contacts, thus limiting the RF current the unit can carry without excessive heating.
The RF20 comprises two fixed contacts supported from opposite ends of a ceramic cylinder, the inner contact being in the form of an annular ring, the outer contact in the form of an annular ring concentric to the inner ring and the axis of the unit. A circular disc supported by a shaft-bearing bellows arrangement, but isolated from it by an insulator, is moved towards the fixed contacts to close the electrical circuit. A compression spring connected to the moveable contact shaft holds the moveable contact away from the fixed contacts. When fluid pressure is exerted on the inside of the bellows, it acts like an air cylinder and exerts a force which overcomes the compression spring and urges the moveable contact disc closed. Although the RF20 also performs very satisfactorily, the unavoidable proximity of the fixed contacts results in high (.apprxeq.10 pf) open contact capacitance and thus high open circuit leakage current at HF and near VHF conditions. Further, the open contact capacitance is relatively insensitive to the position of the moveable contact due to large capacitance between the fixed contacts.
What is desired, therefore, is to provide an improved RF switch having a relatively low capacitance, inductance and resistance associated therewith with high RF current carrying capacity.